How to superimpose a lot of textures in one material in the BGE?
after experimenting different things, I've finally find out how to do it !
To superimpose different textures on one material, you need to go to the very end of the texture panel, in the influence tab you will find the blend option. For the background texture you let this option to "mix" and for your others textures put this option to "add" and it will mix yours textures. (you won't see it in your "texture" viewport shading but you will see it in rendering). If you let the blend option to "mix" for the others textures you need to put them under the bacground texture. This last option won't mix your textures but it will put your last textures on the previous textures. So if you want to see the background texture under the others textures, yours others textures need to have some transparent (alpha) in their images. With these two option you can change the opacity 
of your textures with the difuse:color option of the influence tab.